1. Field of the Invention
The present invention relates to an optical monitoring device that monitors an intensity and a wavelength of light for each wavelength channel in an optical fiber communication using wavelength division multiplexing (WDM), and an optical demultiplexing device incorporated in the optical monitoring device.
2. Description of the Related Art
In conventional WDM transmission schemes in which optical signals of different wavelengths are multiplexed to increase a transmission capacity, a wavelength and a light intensity are monitored by an optical monitoring device for each optical signal channel. FIG. 23 is a schematic of a conventional optical monitoring device. An optical monitoring device 10 includes an optical demultiplexing unit 11 and optical-electrical conversion units 12. The optical-electrical conversion units 12 are provided as many as number of wavelengths multiplexed.
Multiplexed light (hereinafter, “WDM light”) having more than one wavelength is input to input an optical signal from an optical input port “In”. The WDM light is demultiplexed by the optical demultiplexing unit 11 for each wavelength. The optical signal is output to an output port 13 for each channel as single-wavelength light, and is input to the optical-electrical conversion unit 12 to be converted into an electrical signal according to an intensity. The electric signal is output to an output port “Out” as light intensity information for each wavelength.
FIG. 24 is a schematic of the optical demultiplexing unit in the optical monitoring device 10 shown in FIG. 23. As shown, the optical demultiplexing unit 11 is implemented by, for example, an arrayed waveguide grating (AWG). A configuration and a function of the AWG is disclosed in, for example, “IEEE Journal of Selected Topics in Quantum Electronics” volume 2, number 2, June, 1996 pp. 236–250 by Smit, M. K. and Van Dam, C. The optical-electrical conversion unit 12 (12a to 12m) is implemented by, for example, a photodiode. As shown, the optical demultiplexing unit 11 and the optical-electrical conversion unit 12 are integrated on a single substrate 20 in a monolithic manner. Therefore, an assembly work is not required, and a downsized device can be obtained. Such a technology is disclosed in, for example, “Electronics Letters” volume 31, number 7, pp. 581–582, 1995, by M. Ziringible et al., and “IEEE Photonics Technology Letters”, volume 10, number 11, pp. 1614–1616 by M. Kohtoku et al. Such a spectroscopic unit such as the optical demultiplexing unit 11 is formed with a filter allowing light of a specific wavelength range to pass, that is, a filter with a low optical transmission loss of a specific wavelength range.
FIG. 25 is a graph of an optical transmission loss in the spectroscopic unit in the conventional optical monitoring device. A vertical axis represents an optical transmission loss, and a horizontal axis represents a wavelength of transmission light. As shown, the conventional optical demultiplexing unit 11 has an optical-transmission-loss characteristic such that an optical transmission loss of each center wavelength (λ1 to λm) for each channel after demultiplexing is a minimum. That is, even an optical signal demultiplexed for the same channel may have a different optical transmission loss depending on a degree of deviation from the center wavelength, and an error is caused in measurement of intensity of transmission light.
As shown in FIG. 25, when the wavelength of the optical signal is shifted from the center wavelength (for example, ±50 picometers), the transmission loss is changed as expressed by ΔP, thereby reducing transmittance of the optical signal. λA shown in FIG. 25 represents an example of the wavelength of the optical signal input to a channel λm. As such, a phenomenon in which even an optical signal demultiplexed for the same channel has a wavelength shifted from the center wavelength (hereinafter, “wavelength shift”) can be caused. Furthermore, the optical transmission loss characteristic depends on temperature. Therefore, for example, an amount of wavelength shift Δλ in the channel λm with respect to λA varies for each measurement. For this reason, a correction by, for example, setting an amount of error in advance is not applicable. In addition, the conventional optical monitoring device cannot monitor the wavelength of signal light.